We studied human T cell activation and regulation in the system of continued cultures of T cells that respond to T cell growth factor (TCGF). T cells cooperate with monocytes and with B cells during lymphoproliferation to the lectin phytohemagglutinin. Monocytes and a few myeloid and monocytic leukemia cell lines function as accessory cells and produce interleukin-1 (IL-1). We studied the TCGF-producing cells, and cells responding to TCGF, by means of anti-T cell sera, Percoll gradients. Cultured T cells (CTC) are cytotoxic in antibody-dependent cell-mediated cytotoxicity, natural cytotoxicity and specific immune cytolysis. We studied a mouse monoclonal antibody against cultured human T cells and determined that the antibody appears to block the postulated receptor for IL-2 on activated human T cells. We have also tested monoclonal antibodies against the non-polymorphic framework of several molecules within the MHC and shown that portions of HLA-D/DR, HLA-A,B,C and B2m are probably involved in regulating proliferative responses to PWM and OKT3.